Ir/Thioether-Carbene, -Phosphinite, and -Phosphite Complexes for Asymmetric Hydrogenation. A Case for Comparison

© 2019 American Chemical Society. We studied for the first time the potential of novel and simple Ir/thioether-NHC complexes in the asymmetric hydrogenation of unfunctionalized olefins and cyclic β-enamides. For comparison, we prepared and applied the analogues thioether-phosphinite/phosphite complexes. We found that the efficiency of the new Ir/thioether-NHC catalyst precursors varies with the type of olefin. Thus, while the Ir/thioether-NHC catalyst precursors provided lower catalytic performance than their related Ir/thioether-P complexes in the hydrogenation of olefins lacking a coordinating group, the catalysts had similar good performance for the reduction of functionalized olefins (e.g., tri- and disubstituted enol phosphonate derivatives). Catalytic results together with the studies of the reactivity toward H2 indicated that the thioether-carbene design favors the formation of inactive trinuclear species, which are responsible for the low activities obtained with these carbene-type catalysts. Nevertheless, this catalyst deactivation can be avoided by using functionalized olefins such as enol phosphonates. We also report the discovery of simple-to-synthesize Ir/thioether-P catalysts containing a simple backbone that gave high enantioselectivities for some trisubstituted olefins, some challenging 1,1′-disubstituted olefins, and cyclic β-enamides.